The present invention relates to a cutting tool holder for a rotating cutting head in a turning machine for machining cylindrical workpieces, such as pipes, cylindrical steel or the like, the holder being provided to secure a cutting tool having at least one primary and one secondary cutting edge.
Generally, four such holders for cutting tools are mounted on a rotating head of a turning machine. A respective cutting tool is clamped in each of the four holders, which in turn are mounted on the rotating head so that the four cutting tools are symmetrically disposed around the workpiece. Each of the cutting tools has a straight primary cutting edge for rough cutting and a straight secondary cutting edge for finish cutting, the primary cutting edge intersecting the secondary cutting edge at an angle. A cutting tool may have more than one pair of primary and secondary cutting edges so that the tool need not be discarded after the first pair becomes worn. During use a primary and secondary cutting edge pair of each of the four cutting tools disposed around the workpiece simultaneously participate in the cutting, so that a finished shaft is machined in a single process step. The cutting forces should be as symmetrical as possible between the individual cutting tools and the workpiece so that no bending forces are exerted on the head or the workpiece which would interfere with the desired manufacturing precision of the workpiece. For this purpose, the secondary cutting edges, in particular, must be aligned precisely parallel to the center axis of the workpiece.
In a known device, a reversible cutting tool provided with two pairs of primary and secondary cutting edges is inserted in a corresponding recess in a holder and is fastened to the holder by means of a locking or clamping jaw. The holder itself is essentially made of two parts and comprises a basic element provided with a longitudinal groove and an adjustable member which is displaceable therein and whose position can be set relative to the basic element by means of a screw and can be fixed with the aid of a clamping screw that passes through an opening in the basic element.
For economic reasons, the holder and the reversible cutting tool cannot be produced with any desired precision. Since dimensional deviations may add up, it is possible in the known device that an unfavorable angular position results for the secondary cutting edge. Such an unfavorable angular position can be corrected by regrinding the reversible cutting tool, but this leads to undesirable losses in service life.
In a device having a convexly designed cutting edge it is known to adjust the angle of the holder about the center point of a cutting edge which has the shape of a circular arc (see, e.g., German Offenlegungsschrift No. 3,008,547). Such an adjustment, however, cannot be used with cutting tools having linear primary and secondary cutting edges since this would cause the primary cutting edge to be also displaced axially with respect to the cylindrical workpiece, thus producing a thicker--or thinner--chip at this point compared to cutting tools disposed in other holders in the same cutting head, which would interfere with the desired symmetrical distribution of forces.
Furthermore, U.S. Pat. No. 3,842,470 to Karl Hertel discloses a device wherein a reversible cutting tool equipped with two pairs of primary and secondary cutting edges is disposed in a corresponding recess of a holder and can be angularly adjusted by means of a setting wedge. When readjusting the angular position of the secondary cutting edge, the reversible cutting tool is pivoted essentially about its frontal face oriented away from the primary cutting edge when in use, thus causing the point of intersection formed by the primary and secondary cutting edges engaging the workpiece to perform a movement radial to the workpiece. With this known device, it may therefore be necessary, after setting the angle of the secondary cutting edge, to readjust the radial setting. Moreover, this known device has the drawback that, depending on the angular position of the reversible cutting tool, there may occur linear stresses at least at one point between the above-mentioned frontal face of the reversible cutting tool and the corresponding receptacle in the holder.